DC-to-DC converters with high power conversion efficiency are widely used in typical switching power supplies. The DC-to-DC converter is generally configured to drive a switch by a pulse width modulation (“PWM”) method that performs switching at a predetermined switching frequency to prevent current flowing in an inductor from becoming 0 A. This may enhance power conversion efficiency when current continuously flows in a continuous current mode (“CCM”).
However, when a light load such as a device, a circuit or the like is connected to the output of the DC-to-DC converter and supplied with power, the current flowing in the inductor may be 0 A, i.e., a discontinuous current mode (“DCM”), during a partial period of a switching cycle of the PWM method. In this case, since degradation of the power conversion efficiency due to a switching loss increases relatively, the switch is often driven by a pulse frequency modulation (“PFM”) method capable of reducing the number of times of switching.
A burst mode circuit automatically performs switching from a PWM method to a burst mode, such as the PFM method, as disclosed in U.S. Pat. No. 6,307,356, for example. In the burst mode circuit, switching by the PFM method is performed at a fixed duty ratio determined by an oscillator. However, when an input voltage is changed to a low state and the PWM method is set to be switched to the PFM method in a state in which the input voltage is high, for example, the PWM method is switched to the PFM method based on the value set in the high state of the input voltage, thereby causing degradation of power conversion efficiency in the low state of the input voltage. Further, power conversion efficiency is degraded in the same manner not only when the input voltage changes, but also when other conditions change, such as the output voltage, the frequency for switching, the inductance of the inductor, etc.